Various types of downhole power generators have been devised for supplying power to one or more downhole tools, such as sensor tools, measurement-while-drilling (MWD) tools, rotary steerable tools, etc. Many of these downhole generators use fluid power transmitted from the surface to the bottom hole assembly, and commonly rotate a vane within the flow path of the fluid to generate rotary power, which may then be used to generate electrical power. Other devices, such as those disclosed in U.S. Pat. Nos. 6,739,413 and 7,025,152, utilize rotation of a tubular string at the surface to generate downhole power. Those skilled in the art will appreciate that these latter types of systems are not generally favored since rotation of a tubular string may not always be feasible, and may subject the downhole components to high wear.
U.S. Pat. No. 4,415,823 discloses a downhole turbine which drives a generator. U.S. Pat. Nos. 3,036,645 and 2,944,603 also disclose early versions of downhole generators utilizing turbines. U.S. Pat. Nos. 4,369,373, 4,654,537, 4,740,711, 5,149,984, 5,517,464, 5,839,508, 6,672,409, and 7,133,325 also disclose turbine-type devices for generating downhole energy. U.S. Pat. No. 7,002,261 discloses the downhole generation of electrical power utilizing either a turbine or a positive displacement motor, and U.S. Pat. No. 5,248,096 teaches a downhole power generation unit which includes a drilling motor for converting fluid energy into mechanical rotational energy.
U.S. Pat. No. 4,491,738 discloses a technique for generating electrical power downhole with a generator including an anchor which is movable in reciprocating mode in response to pressure pulses in the drilling fluid. U.S. Pat. No. 4,732,225 teaches a downhole motor with a permanent magnet coupling. U.S. Pat. No. 6,011,346 discloses a technique for generating electrical power downhole utilizing piezoelectric members responsive to the flowing stream of fluid.
While various types of downhole generators have been devised, the most popular method of generating power downhole is to use the flowing fluid to rotate a turbine or vane, which then rotates a shaft to drive a generator. Many of these vane-type devices have significant problems due to potential plugging of the device, due to unintentional lost circulation of the fluid, or due to a relatively high rpm but a low torque output. While these vane-type devices have their disadvantages, they also have a significant advantage over other presently available downhole generators, including those which utilize a positive displacement motor. The latter type of prior art devices are believed to suffer from problems associated with articulated joints or universal joints which experience high wear. The bearings on such devices also tend to experience high wear, in part due to the fairly high rpm of the pump in response to fluid flowing through the pump.
The disadvantages of the prior art are overcome by the present invention, and an improved mechanism and technique for generating power downhole is hereinafter disclosed.